Hearts of Darkness

The above is a representation from the film Interstellar, portraying one of the most powerful and mysterious objects in the universe. It is often regarded as a destructive force, one that consumes everything around it. Yet, this intense demonstration of gravity may yet play a part in the very formation of our galaxy and therefore, us. I refer to the black hole.

These enigmatic celestial bodies come in various sizes, and their story is fascinating. They form out of the same violent process that creates neutron stars (supernovae), and from there, they become the point where our understanding of physics breaks down. Black holes are the point where an object reaches infinite density, with a gravitational pull so great, not even light can escape – hence why they are known as black holes – they are invisible, known to exist only by their influence on their surroundings.

So what exactly is a black hole?

A Tipping Point

When I discussed our sun, I mentioned it was a balancing act between gravity and fusion, and eventually, gravity wins. Gravity is the winner here too, but in a more extreme fashion. Stellar mass black holes form when massive stars collapse and the mass at their cores is squeezed to a point of infinite density. One way of describing a black hole is that it’s the child of mass and density – the natural outcome of an intense relationship between the two.

Any matter that falls into a black hole (and any energy) becomes indistinguishable from the rest of the black hole. Objects that get too close are ripped apart and stretched out, ultimately forming a superheated disc of matter around the black hole. These disks reach staggering temperatures, and often the source of powerful X-ray transmissions. A black hole is matter, compressed to a single point, and we have no idea what it’s comprised of, for no information escapes a black hole for us to study.

The popular notion of black holes as galactic vacuums is by and large, wrong. Objects that stray too close will suffer the effects of a black hole’s gravity, but a star and a black hole of equal mass will have a similar effect (gravitationally speaking) on their environment. If our sun collapsed into a black hole tomorrow we’d all die, but not because the earth would get sucked in – instead, a combination of freezing to death and possibly radiation would be what finished us.

The True Giants

Models of how massive stars evolve and die point us to the existence of stellar-mass black holes, but we know of much bigger and more massive black holes. In fact, such a beast rests at the heart of our galaxy.

Sagittarius A* is the location of what is widely accepted to be a supermassive black hole, one that dwarfs every other object in our galaxy.

The formation of this black hole and its relationship with the Milky Way is still a subject of debate – did the galaxy form first, with stars in that cluster near the centre collapsing into black holes that grew and merged? Did a black hole form first, pulling mass into it and stirring up the gas and dust that we now know to be the Milky Way? It’s something of a ‘chicken and egg’ scenario.

In any event, the black hole here is thought to be 4.3 million solar masses. That’s right, it has 4.3 million times the mass of our sun. How do we know this? Through observation.

A number of stars have been observed orbiting around it. These stars would be ripped apart if they got too close (tidal forces would soon see to that), and from the motion of the stars, the mass and diameter can be worked out. The star that gets closest passes within 17 light hours of the centre of the object – to give you an idea of the distance, this over 18 billion kilometres. The closest approach the star makes is roughly 6 times the distance between the sun and the planet Uranus. Since the star does not get ripped apart or collide with this object, its radius must be smaller than this, and in fact, studies suggest a radius of no more than 14 million miles. From the manner of the orbits of the stars around it, the mass can be calculated to the figure above.

The only known object that can cram 4.3 million solar masses into a space that compact is a black hole.

This is not even the biggest supermassive black hole (not by some margin). At the centre of the Centaurus A galaxy is a black hole with 55 million solar masses (that isn’t a typo), but let’s not stop there. The snazzily-named RX J1242-11 galaxy has a black hole with the mass of roughly 100 million suns at its centre.

Can we get bigger? You bet we can. Messier 60 weighs in at 4.5 billion solar masses. The biggest though, is at the heart of galaxy S5 0014+81, is believed to have a mass of 40 billion suns (there is a margin of error in this, due to the intense luminosity of the galactic core).

A Force for Good

Despite the popular representation of black holes as dangerous places (and they are), the effect they have on their surroundings might actually be as positive as it is negative. Their gravity can sweep up gas and dust and compress it, triggering new waves of star formation. They may be responsible for holding galaxies together, which is important to on-going star formation, and the processes associated with star formation (such as planetary formation – in other words, we may owe our existence to a black hole, in some way shape or form). In any event, these are fascinating objects, and ones we are only just beginning to understand.